Flocculation is an essential process for most modern water treatment facilities. In the flocculation process, the water to be treated is introduced to a flocculation tank or basin wherein coagulants or flocculating agents are added to and mixed with water. The coagulants or flocculating agents, e.g., aluminum or iron salts, react with impurities, such as solid inorganic particulates and form large particles, which settle out of the water downstream of the flocculation basin.
Initial flocculation processes typically involved baffled, channel-type flocculation basins. Such flocculation basins typically included around-the-end and over-and-under baffle arrangements. Baffled, channel-type flocculation basins are effective when properly designed but only for appropriate conditions while mixing energy is generally uniform in such flocculation processes, it has been found that optimum flocculation occurs in a narrow range of flow rates. This is because both mixing energy and mixing time depend on the flow rate through the tank. If the flow rate is low, there is high mixing time but low mixing energy. The result is inadequate flocculation. At high flow rates, there is high mixing energy, but low mixing time. This results in the shearing of flocculation particles which do not settle out of the system. In modern water treatment facilities, flow rates are subject to substantial changes--often the highest flow rate being six times greater than the lowest flow rate. As a result, baffled, channel-type arrangements have generally proven to be unsatisfactory.
Many modern flocculation processes now involve mechanical mixers. Such mechanical mixers include perforated panels which rotate or reciprocate in the flocculation tank, rotatable paddles, impeller-type mixers and the like. Such mechanical mixers provide adequate mixing energy, even for low flow rates, however, the mixing energy tends not to be uniform throughout the flocculation tank. Consequently, "short circuiting" occurs, i.e., a portion of the water passing through the flocculation tank passes through areas of low mixing energy and thus receives inadequate mixing time with the flocculation agent.
There is a current need for a flocculation process that provides an optimum amount of mixing energy at all flow rates and eliminates, or at least minimizes flow short circuiting through the process.